Heterodyne radio frequency (RF) receivers are receivers that use one or more internally generated RF signals, referred to as local oscillators (LO), to aid in the reception of a radio signal. The LO signals are generated at a fixed frequency offset relative to the radio frequency of the radio signal to be received. The LO signal is mixed with the received radio signal to produce an “Intermediate Frequency” (IF) at the offset of the LO from the received radio signal. Some heterodyne RF receivers use an IF frequency of zero so that the tuned RF frequency is down-converted to a baseband or DC centered signal. Processing performed at the zero frequency IF is referred to as baseband processing.
The frequency stability of the LO signal within a receiver sets the tuned RF frequency accuracy of the receiver. LO signals are frequently generated with a synthesized RF signal generation process. Synthesized RF signal generation utilizes a frequency reference generator to generate a frequency reference signal that is generally at a relatively low frequency and a radio frequency signal is derived from this reference frequency signal by multiplying the reference frequency by a specified number. The output frequency of a synthesized RF signal generator can be varied during receiver operations by one of two techniques. The output frequency can be changed by reconfiguration of the signal derivation circuitry so that the input signal reference is multiplied by a different number. The output frequency of a synthesized RF signal generator can also be changed by varying the output frequency of the reference generator. Reconfiguration of the signal generation circuitry frequently results in a period of output signal instability before the newly derived output signal becomes stable and usable as an LO within a stable RF receiver. Tracking an RF frequency of a received signal is in many cases performed in a heterodyne receiver by changing the output frequency of the reference frequency oscillator driving the synthesized RF signal generator.
Some radio receivers are required to simultaneously receive multiple RF signals. These receivers often share a common frequency reference generator and use multiple RF synthesizers to generate one or more LO signals for each received signal.
Heterodyne RF receivers frequently track the frequency of the received RF signal or signals. Frequency tracking of the received RF signal is performed to accommodate short and long term frequency instability of both the internally generated LO signal and the RF signal that is being received. Tracking of the received RF signal is typically performed by adjusting the frequency of the LO signal generator so as to properly track the received RF signal. In order to improve reception during LO adjustments, radio receivers with synthesized LO generators sometimes track received signals by adjusting the frequency reference of the LO synthesizer. Radio receivers that receive multiple RF signals and that derive multiple LO signals from a single frequency reference are able to track only one received RF signal by adjustment of the frequency reference. A signal with a highly stable RF frequency is often chosen to be tracked by adjustment of the receiver's frequency reference. The other channels of these receivers typically track signals by changing the LO frequency by reconfiguration of the synthesized signal generator producing the LO signal. When the reference frequency of these receivers is adjusted to track one received signal, the other signals that are tracked by other channels of the RF receiver will be off-tuned from their tuned center frequency. If any other channel changes its tuned center frequency by more than the tracking bandwidth of that channel, the tracking of the received signal is lost. This requires this other channel to reacquire its RF signal, and often introduces periods of signal “drop-outs” where data communicated during this reacquisition is lost. Signals that require lengthy reacquisition processing, such as GPS signals, can be lost for an appreciable amount of time and degrade the usefulness of the receiver.
Therefore a need exists to overcome the problems with the prior art as discussed above.